In recent years, an individual identification technology using wireless communication (hereinafter referred to as a wireless communication system) has attracted attention. In particular, RFID tags (also referred to as IC tags, IC chips, RF tags, wireless tags, or electronic tags) have been useful for production, management, and the like of individual objects. In addition, RFID tags are expected to be used for personal authentification.
The wireless communication system is a system in which data communication is performed with radio signals between a transmitter/receiver serving also as a power supply source or a transmitter/receiver such as a communication device (also referred to as an interrogator or a reader/writer), and a transmitter/receiver such as an RFID tag (hereinafter referred to as an RFID tag).
As for the wireless communication system, research and development for improving efficiency of power supply from a communication device to an RFID tag have been actively conducted in order to extend a communication distance between the communication device and the RFID tag.
In general, power attenuates in proportion to square of the distance from a spot where power is emitted to an observation spot. That is, the longer the distance between a communication device and an RFID tag (hereinafter referred to as a communication distance) is, the smaller amount of power the RFID tag receives, whereas the shorter the communication distance is, the larger amount of power the RFID tag receives.
The amount of power the RFID tag receives is large, which means that the amplitude of a signal received is large. The amount of power the RFID tag receives is small, which means that the amplitude of a signal received is small.
In the case of using an RFID tag, noise might cause a problem (Patent Document 1). Since when the amplitude of the received signal is large, the amount of noise is large, a constant voltage for the RFID tag to operate is fluctuated due to noise and thus a malfunction might occur. Meanwhile, when the amplitude of the received signal is small, the amount of noise is small; however, the received signal itself is significantly adversely affected by momentary external noise, a constant voltage for the RFID tag to operate is fluctuated, and thus a malfunction might occur.